The present invention relates to recycling of vulcanized rubber and is particularly related to a method and apparatus for forming a superfine powder from spent rubber products and waste from rubber production.
It is estimated that 15,000,000 tons or more of vulcanized rubber products are discarded throughout the world annually. Approximately half this amount is in the form of rubber tiresxe2x80x94upwards of 700 million tires are discarded per year worldwide. It is estimated that there are as many as three billion waste tires in the U.S. alone. In addition, a substantial amount of waste rubber is associated with the production of new rubber products. The handling and disposal of this large volume of waste rubber is a significant environmental problem, and the desirability of recycling waste rubber is evident. The improper or unregulated disposal of rubber tires is also known to create fire and health hazards. Unfortunately, the very properties which make vulcanized rubber so useful make it extremely difficult to recycle. Specifically, vulcanized rubber does not melt, age or decompose and is formulated to be used in extreme environmental conditions. Rubber used in tires is highly elastic and tenacious in a temperature range of xe2x88x9250xc2x0-+150xc2x0 C., and is highly resistant to attack by most common chemical substances. While significant research has been devoted to investigating techniques for recycling rubber, to date only a small percentage of waste rubber is recycled.
One avenue of investigation has been directed to forming rubber powders from waste rubber by grinding the rubber. The rubber powders are then used in various ways, for example rubber powder may be added to a raw rubber mix to form new rubber. This technique has important potential cost savings because the rubber powder reduces the energy and processing required to make new rubber. Currently, rubber powder is used in making many rubber products, including tires, hoses, rubber bands, etc. In addition, rubber powders can be added to other products, such as building materials, to improve their properties.
The amount of rubber powder that can be added to a raw rubber mix during rubber production is highly dependant on the size and shape of the rubber particles which comprise the powder. Specifically, it is known that superfine rubber particles (particles having a diameter of about 200 xcexcm or less) may be added in significantly higher percentages than larger particles. However, traditional grinding techniques produce rubber particles having a diameter in excess of 600 xcexcm. To date, addition of rubber powder having particles in excess of 600 xcexcm has not been accepted by producers of rubber products, because rubber made with large particles has poor properties. Rubber powder consisting of particles in the range of 200-400 xcexcm may be added in an amount of up to 10% without adversely affecting the quality of the rubber produced.
Finer grain rubber particles may be produced by grinding the rubber at extremely low (cryogenic) temperatures (for example, using liquid nitrogen) such that the rubber loses its elasticity and grinds more easily. However, due to the energy required to cool the rubber to a sufficiently low temperature, it is uneconomically expensive to form rubber powder in this way.
In Chinese Patent CN 1127693 the inventor of the present invention has previously disclosed a technique for grinding rubber which employs a reagent to cause the rubber to swell. The swollen rubber is less elastic and, therefore, more easily ground into fine particles. The prior disclosure teaches soaking coarsely ground rubber in a non-polar reagent for 5-30 seconds, and then performing further grinding of the resulting swollen powder to achieve a superfine rubber powder. The prior disclosure further teaches that such a rubber powder can be mixed with raw rubber in excess of 40% without impairing the physical properties of the rubber. The prior disclosure suggests the use of toluene, xylene, carbon tetrachloride, carbon trichloride, acetone, ethyl alcohol or mixtures thereof as reagents to cause the rubber to swell.
At the time of the inventor""s prior disclosure, the system for economically creating a superfine powder from waste rubber had not been perfected. Since that time the inventor has developed various improvements and additions to his system, as described herein, which greatly improves the efficiency and usefulness of the system.
Accordingly, one object of the present invention is to provide an optimized system for efficiently creating superfine rubber powder suitable for recycling.
Another object of the present invention is to provide a system for creating a superfine rubber powder with characteristics which allow it to be easily incorporated into new rubber production.
Another object of the present invention is to provide a system for creating superfine rubber powder using a swelling reagent, which provides for highly efficient recycling of the swelling reagent.
The foregoing objects of the invention, and others which will be apparent to those skilled in the art, are realized in the present method and apparatus for making superfine rubber powder from coarsely ground rubber. In one of its broad aspects, the present invention comprises system for forming superfine rubber powder having a soaking container for holding a mixture of coarsely ground rubber particles and a reagent which causes the rubber particles to swell, a grinder for grinding the swollen particles of coarsely ground rubber into superfine particles of rubber, a reagent recovery system comprising a heater and suction equipment for removing the reagent from the superfine particles of rubber to form a dry rubber powder and for recovering the reagent for reuse, and a sorter for classifying the particles of dry rubber according to size. Preferably, the reagent recovery system also collects reagent vapors from the soaking container and from the grinder. The grinder is preferably a slope-faced mill which applies both a substantial grinding force and a substantial shearing force to the coarse rubber particles being ground. The grinder preferably has a stationary face and a rotating face, and at least one of the faces has a plurality of spiral grooves having knife-edges. In one embodiment, the grinder is made of metal and has cooling system to maintain it at a suitable temperature. The separation distance between the faces is, preferably, adjustable. If still finer rubber particles are desired, a second grinder may be used to further grind the reagent-impregnated rubber particles after they are processed by the first grinder. In an automated system, the rubber particles travel from the soaking container to the grinder using a first screw feed or other suitable automated transport mechanism, and from the grinder to the sorter using a second screw feed or transport mechanism. The second screw feed is maintained at an elevated temperature to drive the reagent out of the rubber. The solvent recovery system preferably encloses the entire system, from the container to the second screw feed, and maintains the system at a slight vacuum, so that the reagent is recovered and can, thereafter, be reused.
The present invention also comprises a method of producing superfine rubber powder, comprising the steps of soaking a mixture of a reagent and particles of coarsely ground rubber powder such that the particles become impregnated with the reagent and swell; grinding the reagent-impregnated swollen rubber particles to form superfine rubber particles; heating the superfine rubber particles in an enclosed environment and applying a vacuum pressure to the enclosed environment, such that the reagent vapors are driven from the rubber particles, and collecting and condensing the reagent vapors. A crystalloid salt, preferably one that is compatible with the rubber-making process, such as zinc carbonate, may be added to the swollen rubber particles prior to the grinding step.